1. Field of the Invention
The present invention relates to a method for manufacturing a semiconductor device having a gate insulation film or a dielectric film including a nitrided film, and a semiconductor memory device capable of holding information by storing electric charges in the nitrided film.
2. Description of the Related Art
Recently, an ON film composed of a silicon nitride film formed on a silicon oxide film, and an ONO film composed of a silicon oxide film, a silicon nitride film, and a silicon oxide film formed in this order are used for a memory cell of a semiconductor memory device.
A multilayered insulating film such as the ON film, the ONO film, or the like (hereinafter often referred to as simply a multilayered insulating film) is used as a capacity insulation film with high permittivity and low leakage current in a floating gate type nonvolatile semiconductor memory (hereinafter simply referred to as a floating gate type memory) having an island-shaped floating gate, that is a charge-storage portion, with respect to each memory cell. The multilayered insulating film is used as a gate insulation film of the charge-storage portion in a SONOS type or an MNOS type nonvolatile semiconductor memory (hereinafter simply referred to as a SONOS type (an MNOS type) memory) having the silicon nitride film as the charge-storage portion thereof.
Recently, a tendency toward a demand for miniaturization and high performance of a semiconductor memory has been further increasing, and accordingly, the following serious problems regarding formation of the multilayered insulating film have been raised.
-Influence of Generation of Hydrogen-
A lower silicon oxide film which is on a bottom layer of the ONO film and the ON film in the SONOS type memory, the MNOS type memory, or the like functions as a tunnel oxide film, and extremely high reliability is required. Therefore, generally, it is formed by thermal oxidation of a silicon surface under a temperature condition of 1000° C. or above in dry oxidation, and under a temperature condition of 800° C. or above in wet oxidation.
Subsequently, the silicon nitride film to be the charge-storage portion is formed on the lower silicon oxide film. It is deposited by a thermal CVD method using ammonia and silane as raw material gases. In addition to that high uniformity is required, a temperature condition is set as high as from 700° C. to 900° C. based on the following reasons.
Here, concerning the SONOS type memory, FIG. 36 shows a result of a study in a relationship between a deposition temperature of the silicon nitride film and a threshold value (Vt) shift caused by leaving the silicon nitride film under a high temperature condition. This characteristic chart tells that the higher the deposition temperature of the silicon nitride film is, the lower the amount of Vt shift is, so as to generate a better result. It is inferred that this may be caused by the following reasons.
When the silicon nitride film is formed, a large amount of hydrogen is generated from a raw material gas, and passes into the lower silicon oxide film at the same time with deposition of the silicon nitride film. Simultaneously, the large amount of hydrogen is entrapped also in the silicon nitride film. Here, when the ONO film is formed as the multilayered insulating film, an upper silicon oxide film is formed by further thermal oxidation of a surface of the silicon nitride film. However, since heat treatment with a high temperature and long hours is required, the hydrogen entrapped in the silicon nitride film is diffused and passes into the lower silicon oxide film. It is clear that the passing of the hydrogen into the lower silicon oxide film causes deterioration of a film quality of the lower silicon oxide film.
When the deposition temperature of the silicon nitride film is high, the amount of the hydrogen entrapped in the silicon nitride film decreases. Furthermore, the amount of passing of the hydrogen into the lower silicon oxide film by diffusion is lowered in a later process, which is thought to lower the amount of the Vt shift. Accordingly, the silicon nitride film is required to be formed as high temperature as possible in order to improve the film quality of the lower silicon oxide film by controlling generation of the hydrogen and to obtain the good Vt shift.
The same thing can be said to the floating gate type memory. Since high temperature is required for forming the multilayered insulating film, the hydrogen reaches the lower silicon oxide film through the floating gate, resulting in deterioration of quality of the lower silicon oxide film as the tunnel oxide film.
-Influence of Processing at High Temperature-
As described above, a temperature condition at a high temperature is required when the multilayered insulating film which includes the silicon nitride film functioning as a charge-storage film or a dielectric film is formed, which prevents miniaturization of an element as described below.
In the memory having the multilayered insulating film, when an element isolation structure is formed by, for example, a LOCOS method or an STI (Shallow Trench Isolation) method, the multilayered insulating film is formed after a well is formed by introducing impurities on a substrate. However, the impurities of the well are thermally diffused by the aforementioned processing at high temperature, resulting in difficulty in miniaturization of the element.
Especially, in a memory having sources/drains also serving as embedded bit lines, when the sources/drains are formed after the multilayered insulating film is formed in order to prevent thermal diffusion of the impurities caused by the processing at high temperature, a defect occurs in the multilayered insulating film by ion implantations of the impurities, causing such a problem as increase in leakage current or decrease in reliability.
As described above, even if the multilayered insulating film such as the ON film, the ONO film, or the like is formed by the thermal CVD method or a thermal oxidation method in order to attempt further miniaturization and high performance of the semiconductor memory, the processing at high temperature is required, thereby preventing miniaturization of the element. This makes a current situation that a semiconductor memory of high performance is difficult to be realized.